Cholecalciferol (toxiferol, vitamin D3) is one of the five forms of vitamin D. It is a secosteroid, that is, a steroid molecule with one ring open. This and all forms of vitamin D are misnamed: vitamins by definition are essential organic compounds which cannot be synthesized by the body and must be ingested; cholecalciferol is synthesized by the body, and functions as a prehormone. Cholecalciferol is inactive: it is converted to its active form by two hydoxylations: the first in the liver, the second in the kidney, to form calcitriol, whose action is mediated by the vitamin D receptor, a nuclear receptor which regulates the synthesis of hundreds of enzymes and is present in virtually every cell in the body.
Source: https://en.wikipedia.org/wiki/Cholecalciferol